- To build a centrifuge in space of sufficient size, you need to solve the problem of delivering a large amount of materials to orbit, because it has to be hundreds of meters in diameter at least.
- Such a centrifuge will create a gyroscopic effect, and the station will quickly become very difficult to control.
Unfortunately we have basically no data on the effects of partial gravity, in this context or any other. We can try flying partial-gravity parabolas in aircraft and simulate a Martian toilet the same way they tested the design for Skylab; I don't think this experiment has been done.
It's a significant amount of engineering effort, testing, feedback, and iteration to build effective life support systems for manned spaceflight. Long duration spaceflight is orders of magnitude more difficult.
Toilets are systems that can incapacitate or even kill the crew if they malfunction. In a low or microgravity environment aerosolized septic material can get in astronauts' eyes or lungs. It can also seep into electronics or other ship systems causing malfunctions. Even just clean water spraying into the cabin could be dangerous in microgravity.
If you build a better toilet you need a better pooper to use it. And they need to use it correctly every time or you're going to need a really good waste cleaning and disinfecting strategy for your ship.